All 50 US states have started the process of reopening as President Donald Trump has pushed for, while public health experts have warned that premature moves would lead to more deaths.

Trump said on Thursday that the US would not close if a second wave of COVID-19 comes.

"People say that's a very distinct possibility ... We're not going to close the country. We're going to put out the fires," Trump told reporters during a Ford manufacturing plant tour in the state of Michigan.